Radiographic image processing apparatus, method and recording medium

ABSTRACT

An image obtainment unit obtains plural radiographic-images generated by arranging plural storable-phosphor-sheets in such a manner that at least portions of the sheets are overlapped with each other in an irradiation direction of radiation, by arranging a marker at a position to be detected by the plural sheets, and by detecting the radiation that has passed through a subject and the marker by each of the plural sheets. A first-information obtainment unit obtains first-information representing a distance between detection surfaces of the plural sheets. A second-information obtainment unit obtains second-information representing a magnification ratio of a second marker image of the marker included in a radiographic-image obtained by a second sheet with respect to a marker image of the marker included in a radiographic-image obtained by a first sheet of the plural sheets. A distance calculation unit calculates, based on the first and the second information, a radiation-source-to-image-surface distance.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a divisional application of U.S. applicationSer. No. 15/270,085, filed Sep. 20, 2016, which claims priority under 35U.S.C. § 119 to Japanese Patent Application No. 2015-186580, filed onSep. 24, 2015. The above applications are hereby expressly incorporatedby reference, in their entirety, into the present application.

BACKGROUND

The present disclosure relates to a radiation-source-to-image-surfacedistance obtainment apparatus, method and program that obtains aradiation-source-to-image-surface distance, which is a distance betweena radiation source that outputs radiation to a subject and a detectionsurface of a detection means that detects radiation that has passedthrough a subject. Further, the present disclosure relates to aradiographic image processing apparatus, method and program thatperforms image processing on a radiographic image by using theradiation-source-to-image-surface distance.

Conventionally, in performing radiography on a subject by radiationpassed through the subject, there is a problem that radiation isscattered in the subject, and this scattered radiation (hereinafter,referred to as scattered radiation) lowers the contrast of an obtainableradiographic image. Therefore, radiography is performed, in some cases,by arranging a scattered radiation removal grid (hereinafter, simplyreferred to as a grid) between a subject and a detection means, such asa radiation detector for obtaining a radiographic image by detectingradiation, so that the detection means is not irradiated with thescattered radiation. If radiography is performed by using the grid, thedetection means tends not to be irradiated with radiation scattered bythe subject. Therefore, it is possible to improve the contrast of theradiographic image.

Meanwhile, if radiography is performed by using a grid, a stripe pattern(grid stripe) corresponding to the grid is included in a radiographicimage together with a subject image, and observation of the imagebecomes difficult. Therefore, scattered radiation removal processing inwhich radiography is performed without using a grid, and an effect ofimproving image quality that would be achievable by removal of scatteredradiation by a grid is given to the radiographic image by imageprocessing has been proposed (please refer to Japanese Unexamined PatentPublication No. 2014-207958 (Patent Document 1) and Japanese UnexaminedPatent Publication No. 2015-043959 (Patent Document 2)). Patent Document1 proposes a technique for performing scattered radiation removalprocessing based on virtual grid characteristics, in which a virtualgrid is assumed. Further, Patent Document 2 proposes a technique forperforming scattered radiation removal processing by estimating the bodythickness of a subject.

Further, in recent years, so-called portable radiography, which uses aportable-type radiation irradiation apparatus and a portable-typedetection means, has been performed. The weight of a radiationirradiation apparatus for performing portable radiography has beenreduced to such a degree that an operator can operate the radiationirradiation apparatus held by his/her hand or hands, and it is easy tocarry the radiation irradiation apparatus. Therefore, radiography of asubject is possible at various locations, for example, such as apatient's room in a hospital and a natural disaster site.

SUMMARY

Meanwhile, a radiation-source-to-image-surface distance, i.e. an SID(Source to Image Distance), which is a distance between a radiationsource and a detection surface of a detection means, is needed toperform image processing, such as the aforementioned scattered radiationremoval processing. The SID is easily obtainable at a location, such asa radiography room, in which a radiation source and a detection meansare installed at fixed positions. Therefore, accurate image processingis possible by using the obtained SID.

However, in the aforementioned portable radiography, a radiationirradiation apparatus is used in a state of being held by a hand orhands. Further, a detection means is arranged on the rear side of asubject. Therefore, it is difficult to obtain an accurate SID. In thiscase, an SID may be measured by visual observation, and the obtainedvalue may be used for image processing. However, since the SID measuredby visual observation is not an accurate value, it is impossible toaccurately perform image processing. Further, a sensor or the like formeasuring the SID may be provided in the radiation irradiationapparatus. However, if such a sensor or the like is provided, theconfiguration of the apparatus becomes complex, and also the cost of theapparatus becomes higher.

In view of the foregoing circumstances, the present disclosure isdirected to make it possible to obtain an accurate SID.

Further, the present disclosure is directed to accurately perform imageprocessing by using the SID.

A radiation-source-to-image-surface distance obtainment apparatus of thepresent disclosure includes an image obtainment means that obtainsplural radiographic images generated by arranging plural detection meansthat detect radiation that has been output from a radiation source andpassed through a subject in such a manner that at least portions of theplural detection means are overlapped with each other in an irradiationdirection of the radiation, and by arranging at least one marker at aposition to be detected by the plural detection means, and by detectingthe radiation that has passed through the subject and the marker by eachof the plural detection means, a first information obtainment means thatobtains first information representing a distance between a detectionsurface of a first detection means located closest to the radiationsource among the plural detection means and a detection surface of asecond detection means other than the first detection means among theplural detection means, a second information obtainment means thatobtains second information representing a magnification ratio of asecond marker image of the at least one marker included in a secondradiographic image obtained by the second detection means with respectto a first marker image of the at least one marker included in a firstradiographic image obtained by the first detection means and a distancecalculation means that calculates, based on the first information andthe second information, a radiation-source-to-image-surface distance,which is a distance between the radiation source and the detectionsurface of the first detection means.

As the “detection means”, a storable phosphor sheet as well as aradiation detector may be used. The storable phosphor sheet utilizesstorable phosphor, which stores a part of radiation energy by beingirradiated with radiation, and after then, outputs stimulated emissionlight corresponding to the stored radiation energy by being irradiatedwith excitation light, such as visible light and a laser beam. In thecase that the detection means is a radiation detector, the imageobtainment means should obtain a radiographic image represented by imagesignals output from the radiation detector. In the case that thedetection means is a storable phosphor sheet, a radiography apparatus isused, and radiographic image information is temporarily stored andrecorded on a storable phosphor sheet by irradiation of the storablephosphor sheet with radiation that has passed through a subject.Further, an image readout apparatus is used, and stimulated emissionlight is induced by irradiation of this storable phosphor sheet withexcitation light, and image signals representing a radiographic image isgenerated by performing photoelectric conversion on the stimulatedemission light. Therefore, the image obtainment means should obtain aradiographic image represented by image signals generated in thismanner.

The expression “arranging plural detection means in such a manner thatat least portions of the plural detection means are overlapped with eachother in an irradiation direction of the radiation” means that detectionsurfaces of the plural detection means are arranged perpendicular to anoptical axis of radiation output from a radiation source, and that atleast portions of the plural detection means are overlapped with eachother. In this case, the plural detection means may be in close contactwith each other or spaced apart from each other at the overlappedportions. Further, the expression “arranging in such a manner that atleast portions of the plural detection means are overlapped with eachother” includes both of a case in which the plural detection means arearranged in such a manner that the entire areas of the plural detectionmeans are overlapped with each other and a case, such as long-sizeradiography, in which the plural detection means are arranged in such amanner that portions of the plural detection means are overlapped witheach other.

The “marker” has a shape extending in the surface direction of thedetection means. For example, the marker may have a cross shape ofintersecting segments. Further, the marker is made of arbitrary materialthat is able to make a marker image, which is an image of the marker,included in a radiographic image in such a manner to be distinguishablefrom a subject image. In the case that a contrast between the tissue ofthe subject and the marker is considered, it is desirable that themarker is made of material, such as metal that does not pass radiation.Here, it is desirable that the marker is in close contact with thedetection surface of the first detection means. However, the marker maybe arranged away from the detection surface of the first detectionmeans.

In the radiation-source-to-image-surface distance obtainment apparatusof the present disclosure, in the case that the arranged at least onemarker is plural markers, the second information obtainment means mayobtain, as the second information, an average value of the magnificationratio obtained for each marker.

A radiographic image processing apparatus of the present disclosureincludes the radiation-source-to-image-surface distance obtainmentapparatus of the present disclosure, and an image processing means thatperforms image processing on at least one of the plural radiographicimages by using the radiation-source-to-image-surface distance obtainedby the radiation-source-to-image-surface distance obtainment apparatus.

In the radiographic image processing apparatus of the presentdisclosure, the image processing means may perform scattered radiationremoval processing, as the image processing.

A radiation-source-to-image-surface distance obtainment method of thepresent disclosure includes obtaining plural radiographic imagesgenerated by arranging plural detection means that detect radiation thathas been output from a radiation source and passed through a subject insuch a manner that at least portions of the plural detection means areoverlapped with each other in an irradiation direction of the radiation,and by arranging at least one marker at a position to be detected by theplural detection means, and by detecting the radiation that has passedthrough the subject and the marker by each of the plural detectionmeans, obtaining first information representing a distance between adetection surface of a first detection means located closest to theradiation source among the plural detection means and a detectionsurface of a second detection means other than the first detection meansamong the plural detection means, obtaining second informationrepresenting a magnification ratio of a second marker image of the atleast one marker included in a second radiographic image obtained by thesecond detection means with respect to a first marker image of the atleast one marker included in a first radiographic image obtained by thefirst detection means, and calculating, based on the first informationand the second information, a radiation-source-to-image-surfacedistance, which is a distance between the radiation source and thedetection surface of the first detection means.

A radiographic image processing method of the present disclosureincludes obtaining the radiation-source-to-image-surface distance byusing the radiation-source-to-image-surface distance obtainment methodof the present disclosure, and performing image processing on at leastone of the plural radiographic images by using theradiation-source-to-image-surface distance.

Here, the radiation-source-to-image-surface distance obtainment methodand the radiographic image processing method of the present disclosuremay be provided as programs to be executed by a computer.

According to the present disclosure, first information representing adistance between a detection surface of a first detection means locatedclosest to the radiation source among plural detection means and adetection surface of a second detection means other than the firstdetection means among the plural detection means is obtained. Further,second information representing a magnification ratio of a second markerimage of at least one marker included in a second radiographic imageobtained by the second detection means with respect to a first markerimage of the at least one marker included in a first radiographic imageobtained by the first detection means is obtained. Further, aradiation-source-to-image-surface distance is calculated based on thefirst and second information. Therefore, it is possible to obtain anaccurate radiation-source-to-image-surface distance by using anapparatus of simple and low-cost configuration without providing asensor and the like.

Further, image processing is performed on at least one of pluralradiographic images by using the radiation-source-to-image-surfacedistance. Therefore, it is possible to perform highly accurate imageprocessing by using the accurate radiation-source-to-image-surfacedistance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram illustrating the configuration of aradiography system to which a radiation-source-to-image-surface distanceobtainment apparatus and a radiographic image processing apparatusaccording to embodiments of the present disclosure have been applied;

FIG. 2 is a schematic diagram illustrating the configuration of an imagereadout apparatus;

FIG. 3 is a schematic diagram illustrating the configuration of aradiographic image processing apparatus realized by installing aradiographic image processing program in a computer;

FIG. 4A is a diagram illustrating a first radiographic image;

FIG. 4B is a diagram illustrating a second radiographic image;

FIG. 5 is a diagram for explaining calculation of an SID in energysubtraction radiography;

FIG. 6 is a block diagram illustrating scattered radiation removalprocessing;

FIG. 7 is a flow chart showing processing performed in embodiments ofthe present disclosure;

FIG. 8 is a diagram for explaining long-size radiography; and

FIG. 9 is a diagram for explaining calculation of an SID in long-sizeradiography.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, embodiments of the present disclosure will be describedwith reference to drawings. FIG. 1 is a schematic block diagramillustrating the configuration of a radiography system to which aradiation-source-to-image-surface distance obtainment apparatus and aradiographic image processing apparatus according to embodiments of thepresent disclosure have been applied. As illustrated in FIG. 1, theradiography system according to embodiments of the present disclosure isa system for radiographing a radiographic image of subject H andperforming, on the radiographic image, various kinds of image processingincluding scattered radiation removal processing. The radiography systemincludes a radiography apparatus 1, an image readout apparatus 2, and acomputer 3 including a radiation-source-to-image-surface distanceobtainment apparatus and a radiographic image processing apparatusaccording to embodiments of the present disclosure.

The radiography apparatus 1 is a radiography apparatus for performingso-called one-shot energy subtraction, in which two storable phosphorsheets IP1, IP2 are irradiated, at different energy from each other,with X-rays that have been output from an X-ray source 4, which is aradiation source, and passed through subject H. When radiography isperformed, first storable phosphor sheet IP1 and second storablephosphor sheet IP2 are arranged in this order from a side closer to theX-ray source 4, as illustrated in FIG. 1. Further, a filter 5 for X-rayenergy conversion, which is composed of a copper plate, is arrangedbetween these two sheets IP1, IP2, and the X-ray source 4 is driven.Here, storable phosphor sheets IP1, IP2 and the filter 5 for X-rayenergy conversion are in close contact with each other.

Accordingly, first storable phosphor sheet IP1 stores and recordsradiographic image information about subject H by low-voltage X-rays,which also include so-called soft radiation, and second storablephosphor sheet IP2 stores and records radiographic image informationabout subject H by high-voltage X-rays after the soft radiation has beenremoved. At this time, the positional relationship of subject H withstorable phosphor sheet IP1 is the same as the positional relationshipof subject H with storable phosphor sheet IP2. Accordingly, radiographicimage information in which at least a part of image information aboutsubject H is different from each other is stored and recorded in twostorable phosphor sheets IP1, IP2. Meanwhile, in the present embodiment,plural markers 6 made of metal, such as lead, that does not pass X-raysare arranged on a detection surface of first storable phosphor sheetIP1. Here, storable phosphor sheets IP1, IP2 correspond to detectionmeans.

FIG. 2 is a schematic diagram illustrating the configuration of an imagereadout apparatus. First, while storable phosphor sheet IP1 of storablephosphor sheets IP1 IP2, in which radiographic image information hasbeen stored and recorded as described above, is moved by an endless belt9 in the direction of arrow Y, main scan is performed in X direction onsheet IP1 with excitation light 11, which is a laser beam from a laserlight source 10, by deflecting the excitation light 11 by a scan mirror12. Stimulated emission light 13 of a light amount corresponding toradiographic image information stored and recorded in storable phosphorsheet IP1 is emitted from storable phosphor sheet IP1 by scan with theexcitation light. The stimulated emission light 13 enters the inside ofa light guide 14 from an end surface of the light guide 14 produced byforming a transparent acrylic plate, and travels in the inside of thelight guide 14 while repeating total reflection, and is received by aphotomultiplier 15. The photomultiplier 15 outputs analog output signalQ1 corresponding to the light emission amount of the stimulated emissionlight 13, in other words, representing radiographic image informationabout subject H.

A conversion unit 16 performs logarithmic transformation on outputsignal Q1, and further A/D conversion on the signal after logarithmictransformation. Accordingly, output signal Q1 is converted to digitalfirst radiographic image G1. Next, output signal Q2 is obtained byreading out image information recorded in the other storable phosphorsheet IP2, and the conversion unit 16 converts output signal Q2 todigital second radiographic image G2 exactly in a similar manner. Firstand second radiographic images G1, G2 are input to the computer 3.

A display unit 18 and an input unit 19 are connected to the computer 3.The display unit 18 includes a CRT (Cathode Ray Tube), a liquid crystaldisplay or the like, and displays a radiographic image obtained byradiography, and assists a user in various kinds of input necessary forprocessing performed in the computer 3. The input unit 19 includes akeyboard, a mouse, a touch panel or the like.

A radiation-source-to-image-surface distance obtainment program and aradiographic image processing program according to the embodiments ofthe present disclosure have been installed in the computer 3. Since theradiation-source-to-image-surface distance obtainment program isincluded in the radiographic image processing program, theradiation-source-to-image-surface distance obtainment program and theradiographic image processing program will be simply referred tohereafter as the radiographic image processing program. In theembodiments of the present disclosure, a computer may be a workstationor a personal computer directly operated by an operator, or a servercomputer connected to them through a network. The radiographic imageprocessing program is recorded in a recording medium, such as a DVD(Digital Versatile Disc) and a CD-ROM (Compact Disc Read Only Memory),and distributed, and installed in a computer from the recording medium.Alternatively, the program is stored in a storage device of a servercomputer connected to a network or in a network storage in an accessiblemanner from the outside, and downloaded to a computer by a request, andinstalled.

FIG. 3 is a schematic diagram illustrating the configuration of aradiographic image processing apparatus realized by installing aradiographic image processing program in the computer 3. As illustratedin FIG. 3, the radiographic image processing apparatus includes, asstandard computer configuration, a CPU (Central Processing Unit) 21, amemory 22, and a storage 23.

The storage 23 includes a storage device, such as a hard disk and an SSD(Solid State Drive). Various kinds of information including a programfor driving each unit of the radiography apparatus 1 and theradiographic image processing program have been stored in the storage23. Further, radiographic images obtained by radiography are also storedin the storage 23. Further, various kinds of table that will bedescribed later are stored in the storage 23.

A program stored in the storage 23, or the like is temporarily stored inthe memory 22 to cause the CPU 21 to perform various kinds ofprocessing. The radiographic image processing program defines, asprocessing to be performed by the CPU 21, image obtainment processingfor obtaining first and second radiographic images G1, G2, firstinformation obtainment processing for obtaining first informationrepresenting a distance between a detection surface of first storablephosphor sheet IP1 located closest to the X-ray source 4 and a detectionsurface of second storable phosphor sheet IP2, second informationobtainment processing for obtaining second information representing amagnification ratio of a second marker image of a marker 6 included insecond radiographic image G2 obtained by second storable phosphor sheetIP2 with respect to a first marker image of the marker 6 included infirst radiographic image G1 obtained by first storable phosphor sheetIP1, distance calculation processing for calculating, based on the firstand second information, an SID, which is a distance between the X-raysource 4 and first storable phosphor sheet IP1, image processing onradiographic images G1, G2 by using the SID, and energy subtractionprocessing on processed radiographic images G1, G2.

The CPU 21 performs these kinds of processing based on the radiographicimage processing program, and thereby the computer 3 functions as animage obtainment unit 31, a first information obtainment unit 32, asecond information obtainment unit 33, a distance calculation unit 34,an image processing unit 35, and an energy subtraction processing unit36. Here, the computer 3 may include processors for performing imageobtainment processing, first information obtainment processing, secondinformation obtainment processing, distance calculation processing,image processing, and energy subtraction processing, respectively. Here,storable phosphor sheets IP1, IP2 correspond to first and seconddetection means, respectively. Further, the image obtainment unit 31,the first information obtainment unit 32, the second informationobtainment unit 33, and the distance calculation unit 34 constitute asource-to-image distance obtainment apparatus of the present disclosure.

The image obtainment unit 31 obtains first and second radiographicimages G1, G2 generated by the image readout apparatus 2, and storesfirst and second radiographic images G1, G2 in the storage 23. Here, inthe case that first and second radiographic images G1, G2 have beenstored in another storage device, such as a server, first and secondradiographic images G1, G2 should be obtained from the storage device.

The first information obtainment unit 32 obtains first information J1representing distance D between a detection surface of first storablephosphor sheet IP1 and a detection surface of second storable phosphorsheet IP2. Here, since the thicknesses of first and second storablephosphor sheets IP1, IP2 and the thickness of the filter 5 are alreadyknown, distance D is a value obtained by adding the thickness of thefilter 5 to the thickness of first storable phosphor sheet IP1. Thefirst information obtainment unit 32 obtains first information J1representing distance D by receiving an input by an operator from theinput unit 19. Alternatively, first information J1 may have been storedin the storage 23 in advance. In this case, the first informationobtainment unit 32 obtains first information J1 by reading out firstinformation J1 from the storage 23.

The second information obtainment unit 33 obtains second information J2representing a magnification ratio of a second marker image of a marker6 included in second radiographic image G2, obtained by second storablephosphor sheet IP2, with respect to a first marker image of the marker 6included in first radiographic image G1, obtained by first storablephosphor sheet IP1. FIGS. 4A and 4B are diagrams illustrating first andsecond radiographic images G1, G2. As illustrated in FIGS. 4A and 4B,first and second radiographic images G1, G2 include image informationabout subject H and plural marker images M1, M2 of the markers 6. Here,the marker has a shape in which a circular frame and cross-shapedintersecting segments are combined together.

Meanwhile, X-rays output from the X-ray source 4 are cone beams.Therefore, the size of second marker image M2 included in secondradiographic image G2 is larger than the size of first marker image M1included in first radiographic image G1. The second informationobtainment unit 33 obtains, as second information J2, magnificationratio K of second marker image M2 with respect to first marker image M1.In the present embodiment, four markers 6 are used. Therefore, fourfirst marker images M1 and four second marker images M2 are included infirst and second radiographic image G1, G2, respectively. The secondinformation obtainment unit 33 calculates four magnification ratios forthe markers 6, respectively, and obtains, as second information J2,magnification ratio K that is a representative value, such as an averageor a median of the four magnification ratios. Magnification ratio K iscalculated by template matching between first marker image M1 and secondmarker image M2 corresponding to each other. Alternatively, a ratio ofdiameters of circular parts of first and second marker images M1, M2 ina predetermined direction may be used.

The distance calculation unit 34 calculates an SID based on firstinformation J1 and second information J2. FIG. 5 is a diagram forexplaining calculation of an SID. In FIG. 5, a broken line indicates aportion of X-rays output from the X-ray source 4 that irradiates themarker 6. The range of first marker image M1 is a range in which firststorable phosphor sheet IP1 and the broken line portion of the X-rayscross each other. The range of second marker image M2 is a range inwhich second storable phosphor sheet IP2 and the broken line portion ofthe X-rays cross each other. Magnification ratio K that is secondinformation J2 obtained by the second information obtainment unit 33 maybe expressed by the following expression (1), using an SID and distanceD between the detection surface of first storable phosphor sheet IP1 andthe detection surface of second storable phosphor sheet IP2:K=(SID+D)/SID  (1).

Therefore, the SID is calculable by using the following expression (2),using distance D and magnification ratio K:SID=D/(K−1)  (2).

For example, in the case that D is 2 cm, and magnification ratio K is1.01, the SID is 200 cm.

The image processing unit 35 performs image processing on first andsecond radiographic image G1, G2 by using the SID. In the presentembodiment, image processing including scattered radiation removalprocessing is performed. Next, scattered radiation removal processingwill be described. FIG. 6 is a block diagram illustrating scatteredradiation removal processing;

In the present embodiment, no grid is used during radiography.Therefore, the image processing unit 35 performs scattered radiationremoval processing on first and second radiographic images G1, G2 togive a similar scattered radiation removal effect achievable ifradiography is performed by actually using a grid. Scattered radiationremoval processing is performed by using virtual grid characteristicsthe actual use of which is presumable, for example, as described inPatent Document 1. Therefore, the image processing unit 35 obtainsvirtual grid characteristics by an input by an operator from the inputunit 19. In the present embodiment, the virtual grid characteristics arescattered radiation transmittance Ts about a virtual grid andtransmittance of primary radiation irradiating first and second storablephosphor sheet IP1, IP2 after passing through subject H (primaryradiation transmittance) Tp. Here, scattered radiation transmittance Tsand primary radiation transmittance Tp are values between 0 and 1.

The image processing unit 35 may obtain the virtual grid characteristicsby directly receiving inputs of the values of scattered radiationtransmittance Ts and primary radiation transmittance Tp. However, in thepresent embodiment, the virtual grid characteristics, i.e, scatteredradiation transmittance Ts and primary radiation transmittance Tp areobtained by receiving an input of radiography conditions from the inputunit 19 at the time of obtainment of a radiographic image.

Radiography conditions include an SID, the radiation dose ofradiography, tube voltage, the material of a target of a radiationsource and a filter, the kind of a storable phosphor sheet used inradiography, and the like. Here, in radiography of a radiographic image,the kind of a grid to be used has been generally determined based onradiography conditions, and scattered radiation transmittance Ts andprimary radiation transmittance Tp are different based on the kind ofthe grid. Therefore, regarding radiography conditions, a table showingcorrespondence between various kinds of radiography conditions andvirtual grid characteristics has been stored in the storage 23.Meanwhile, the various kinds of radiography conditions have been oftendetermined based on facilities in which a radiography system isinstalled. Therefore, in the case that radiography conditions duringactual radiography are unknown, radiography conditions based on thefacilities should be used. The image processing unit 35 obtains, withreference to the table stored in the storage 23, virtual gridcharacteristics based on radiography conditions input from the inputunit 19.

Further, the image processing unit 35 calculates, based on the followingexpressions (3), (4), a primary radiation image and a scatteredradiation image from distribution T(x, y) of the thickness of a subjectin radiographic images G1, G2. Then, the image processing unit 35calculates, based on expression (5), distribution S (x, y) of scatteredradiation contents from the calculated primary radiation image andscattered radiation image:Icp(x,y)=Io(x,y)×exp(−μ×T(x,y))  (3);Ics(x,y)=Io(x,y)*Sσ(T(x,y))  (4); andS(x,y)=Ics(x,y)/(Ics(x,y)+Icp(x,y))  (5),

where (x, y) is the coordinate of a pixel position of projection imageGi,

Icp(x, y) is a primary radiation image at pixel position (x, y),

Ics(x, y) is a scattered radiation image at pixel position (x, y),

Io(x, y) is an incident radiation dose onto a subject surface at pixelposition (x, y),

μ is a radiation attenuation coefficient of subject H, and

Sσ(T(x, y)) is convolution kernel representing the characteristics ofscatter based on the thickness of a subject at pixel position (x, y).

Further, distribution T(x, y) of the thickness of a subject should becalculated by assuming that the distribution of brightness in first andsecond radiographic images G1, G2 substantially coincides with thedistribution of the thickness of a subject, and by converting pixelvalues of first and second radiographic images G1, G2 to thicknesses bythe value of radiation attenuation coefficient. Alternatively, thethickness of subject H may be measured by using a sensor or the like, orapproximated by a model, such as a cube and an elliptical cylinder.

Incident radiation dose Io(x, y) is the dose of X-rays irradiatingstorable phosphor sheets IP1, IP2 when it is assumed that subject H isnot present. Incident radiation dose Io(x, y) changes based on the SID,the tube voltage and an mAs value. In the present embodiment, a tableshowing correspondence between various kinds of SID's, tube voltages andmAs values and incident radiation dose has been stored in the storage23. Further, incident radiation dose Io(x, y) is obtained, withreference to this table, based on the SID, the tube voltage and the mAsvalue.

In Expression (4), * is an operator denoting a convolution operation.Further, Sσ(T(x, y)) may be experimentally obtained based on radiographyconditions. In the present embodiment, a table showing correspondencebetween various radiography conditions and Sσ(T(x, y)) has been storedin the storage 23, and Sσ(T(x, y)) is obtained, with reference to thistable, based on radiography conditions.

Further, the image processing unit 35 calculates conversion coefficientR(x, y) for converting radiographic images G1, G2 based on scatteredradiation transmittance Ts and primary radiation transmittance Tp, whichare virtual grid characteristics, and distribution S(x, y) of scatteredradiation contents by the following expression (6). Further, the imageprocessing unit 35 multiplies the pixel value of each pixel in first andsecond radiographic image G1, G2 by conversion coefficient R(x, y) bythe following expression (7), thereby obtaining first and secondprocessed radiographic images by removing scattered radiation componentsfrom first and second radiographic images G1, G2:R(x,y)=S(x,y)×Ts+(1−S(x,y))×Tp  (6); andGs(x,y)=R(x,y)×G(x,y)  (7).

Here, first and second radiographic images G1, G2 may be decomposed intoplural frequency bands, and a conversion coefficient may be calculatedfor each of the frequency bands, and multiplication processing using theconversion coefficient may be performed for each of the frequency bands.In this case, processed first and second radiographic images Gs1, Gs2are obtained by performing frequency synthesis on projection images ofrespective frequency bands multiplied by conversion coefficients.

Further, the image processing unit 35 may also perform other imageprocessing, such as gradation correction processing, density correctionprocessing, and frequency emphasis processing, on processed radiographicimages Gs1, Gs2.

The energy subtraction processing unit 36 performs weighted subtractionprocessing between corresponding pixels in processed radiographic imageGs1, Gs2. Accordingly, the energy subtraction processing unit 36generates a soft region image, in which only a soft region of subject Hhas been extracted, and a bone region image, in which only a bone regionof subject H has been extracted. In this case, registration of processedradiographic image Gs1, Gs2 is performed by using marker images M1, M2.Specifically, registration should be performed by performing paralleltranslation, rotation, and enlargement or reduction on at least one ofprocessed radiographic images Gs1, Gs2 so that marker images M1, M2match with each other.

Next, processing performed in the embodiments of the present disclosurewill be described. FIG. 7 is a flow chart showing processing performedin the embodiments of the present disclosure. First, the imageobtainment unit 31 obtains first and second radiographic images G1, G2that have been generated from first and second storable phosphor sheetsIP1, IP2 by the image readout apparatus 2 (step ST1). Then, the firstinformation obtainment unit 32 obtains first information J1 representingdistance D between a detection surface of first storable phosphor sheetIP1 and a detection surface of second storable phosphor sheet IP2 (stepST2). Further, the second information obtainment unit 33 obtains secondinformation J2 representing magnification ratio K of a second markerimage of a marker 6 included in second radiographic image G2, obtainedby second storable phosphor sheet IP2, with respect to a first markerimage of the marker 6 included in first radiographic image G1, obtainedby first storable phosphor sheet IP1 (step ST3).

Further, the distance calculation unit 34 calculates an SID based onfirst information J1 and second information J2 (step ST4). Then, theimage processing unit 35 obtains processed radiographic images Gs1, Gs2by performing image processing including scattered radiation removalprocessing on first and second radiographic image G1, G2 (step ST5).Further, the energy subtraction processing unit 36 generates a softregion image, in which only a soft region of subject H has beenextracted, and a bone region image, in which only a bone region ofsubject H has been extracted, by performing energy subtractionprocessing on processed radiographic images Gs1, Gs2 (step ST6), andprocessing ends. The soft region image and the bone region image aredisplayed on the display unit 18, and provided for diagnosis.

In this way, in the embodiments of the present disclosure, the SID iscalculated based on first information J1 and second information J2.Therefore, it is possible to obtain an accurate SID by using a simpleand low-cost apparatus without providing a sensor or the like.

Further, it is possible to perform high-accuracy image processing usingan accurate SID by performing image processing on plural first andsecond radiographic image G1, G2 using the obtained SID.

In the above embodiment, radiographic images of subject H are obtainedby stacking two storable phosphor sheet IP1, IP2 in order to performenergy subtraction processing. It is also possible to calculate an SIDby using the technique of the present application by using radiographicimages obtained by long-size radiography for a long-size region, such asthe whole bone (the whole spine) or the whole leg (the whole lower limb)of subject H, as a radiography target. FIG. 8 is a diagram forexplaining long-size radiography. In FIG. 8, three storable phosphorsheets IP1, IP2, IP3 are used. As illustrated in FIG. 8, three storablephosphor sheets IP1, IP2, IP3 are arranged in such a manner to beoverlapped with each other during long-size radiography. Further, amarker 6 is arranged on a surface of storable phosphor sheet IP1 ofstorable phosphor sheets IP1, IP3, which are closer to the X-ray source4. In long-size radiography, radiographic information about the wholebody of subject H is stored and recorded on arranged three storablephosphor sheets IP1, IP2, IP3, and radiographic images G1, G2, G3 aregenerated by reading out radiographic image information from storablephosphor sheets IP1, IP2, IP3, respectively. Further, long-sizeradiographic image GL is generated by connecting generated threeradiographic images G1, G2, G3 together.

Here, overlapped parts of radiographic image G1, G2 of radiographicimages G1, G2, G3 obtained by long-size radiography include markerimages M1, M2 of the marker 6, respectively. FIG. 9 is a diagram forexplaining calculation of an SID in long-size radiography. In FIG. 9, abroken line indicates a portion of X-rays output from the X-ray source 4that irradiates the marker 6. The range of first marker image M1 is arange in which first storable phosphor sheet IP1 and the broken lineportion of the X-rays cross each other. The range of second marker imageM2 is a range in which second storable phosphor sheet IP2 and the brokenline portion of the X-rays cross each other. As illustrated in FIG. 9,the relationship among magnification ratio K of second marker image M2with respect to first marker image M1, distance D between detectionsurfaces of storable phosphor sheets IP1, IP2 and SID is the same as therelationship about the case illustrated in FIG. 5. Therefore, also inthe case that long-size radiography has been performed, the firstinformation obtainment unit 32 obtains, as first information J1,distance D between detection surfaces of storable phosphor sheets IP1,IP2, and the second information obtainment unit 33 obtains, as secondinformation J2, magnification ratio K of second marker image M2 withrespect to first marker image M1, thereby the distance calculation unit34 is able to calculate the SID by using the aforementioned expression(2).

In the aforementioned embodiments, the storable phosphor sheets areirradiated with X-rays that have passed through subject H, andradiographic images are obtained by reading radiographic informationfrom the storable phosphor sheets at the image readout apparatus 2.Alternatively, radiographic images may be obtained by using radiationdetectors instead of the storable phosphor sheets.

The radiation detector is able to repeat recording and readout ofradiographic images. A so-called direct-type radiation detector, whichgenerates charges by being directly irradiated with radiation, may beused. Alternatively, a so-called indirect-type radiation detector, whichtemporarily converts radiation into visible light, and converts thevisible light into electric charge signals, may be used. Further, it isdesirable to use, as a method for reading out radiographic imagesignals, a so-called TFT readout method, in which radiographic imagesignals are read out by ON/OFF of a TFT (thin film transistor) switch,or a so-called light readout method, in which radiographic image signalsare read out by irradiation with readout light. However, the method isnot limited to these methods, and other methods may be used.

Here, also in the case that the radiation detectors are used, an SID iscalculable in a similar manner to the aforementioned embodiments. Here,first and second radiation detectors DR1, DR2 are used instead of firstand second storable phosphor sheets IP1, IP2, used in the aforementionedembodiments, and first and second radiographic images G1, G2 areobtained from first and second radiation detectors DR1, DR2,respectively. The first information obtainment unit 32 obtains firstinformation J1 representing a distance between a detection surface offirst radiation detector DR1 and a detection surface of second radiationdetector DR2. Also in this case, the thicknesses of first and secondradiation detectors DR1, DR2, and the thickness of a filter 5 arealready known. Therefore, distance D is a value obtained by adding thethickness of the filter 5 to the thickness of first radiation detectorDR1. Meanwhile, the second information obtainment unit 33 obtains secondinformation J2, which is a magnification ratio of a second marker imageof a marker 6 included in second radiographic image G2, obtained bysecond radiation detector DR2, with respect to a first marker image ofthe marker 6 included in first radiographic image G1, obtained by firstradiation detector DR1. Further, the distance calculation unit 34 isable to calculate an SID based on first information J1 and secondinformation J2 by using the abovementioned expression (2).

In the aforementioned embodiments, plural markers 6 are used.Alternatively, only one marker 6 may be used.

Further, in the aforementioned embodiments, a marker 6 made of metalthat does not pass X-rays is used. Alternatively, a marker 6 made ofmaterial, such as metal or resin, that passes X-rays may be used as longas it is possible to make a marker image included in a radiographicimage in such a manner to be distinguishable from a subject image.

Further, in the aforementioned embodiments, a marker 6 is placed inclose contact with a storable phosphor sheet that is closer to the X-raysource 4. Alternatively, the marker 6 may be arranged away from thestorable phosphor sheet. For example, the marker 6 may be arranged, forexample, on subject H.

Further, in the aforementioned embodiments, radiography is performed byarranging two storable phosphor sheets in such a manner to be overlappedwith each other with respect to the optical axis of X-rays.Alternatively, radiography may be performed by arranging three or morestorable phosphor sheets in such a manner to be overlapped with eachother. Also in that case, an SID is calculable in a similar manner tothe aforementioned embodiments. In this case, distance D between adetection surface of a storable phosphor sheet closest to the X-raysource and a detection surface of one of second closest and laterstorable phosphor sheets, and magnification ratio K of a marker includedin a radiographic image obtained from the one of the storable phosphorsheets used in obtainment of the aforementioned distance D with respectto a marker image included in a radiographic image obtained from thestorable phosphor sheet closest to the X-ray source are obtained, andused to calculate an SID.

In the aforementioned embodiments, the technique disclosed in PatentDocument 1 is used, as scattered radiation removal processing, but anarbitrary technique, such as a technique disclosed in Patent Document 2,may be used.

In the aforementioned embodiments, scattered radiation removalprocessing is performed as processing using an SID, but other imageprocessing using an SID may be performed.

What is claimed is:
 1. A radiographic image processing apparatus comprising: an image obtainment unit that obtains a plurality of radiographic images generated by arranging a plurality of detection unit that detect radiation that has been output from a radiation source and passed through a subject in such a manner that at least portions of the plurality of detection unit are overlapped with each other in an irradiation direction of the radiation, and by arranging at least one marker at a position to be detected by the plurality of detection unit, and by detecting the radiation that has passed through the subject and the marker by each of the plurality of detection unit; a first information obtainment unit that obtains first information representing a distance between a detection surface of a first detection unit located closest to the radiation source among the plurality of detection unit and a detection surface of a second detection unit other than the first detection unit among the plurality of detection unit; a second information obtainment unit that obtains second information representing a magnification ratio of a second marker image of the at least one marker included in a second radiographic image obtained by the second detection unit with respect to a first marker image of the at least one marker included in a first radiographic image obtained by the first detection unit; a distance calculation unit that calculates, based on the first information and the second information, a radiation-source-to-image-surface distance, which is a distance between the radiation source and the detection surface of the first detection unit; an image processing unit that performs scattered radiation removal processing on at least one of the plurality of radiographic images by using the radiation-source-to-image-surface distance; and an energy subtraction processing unit that performs energy subtraction processing on the plurality of radiographic images which include the at least one radiographic image on which the scattered radiation removal processing is performed.
 2. The radiographic image processing apparatus, as defined in claim 1, wherein the second information obtainment unit obtains, as the second information, an average value of the magnification ratio obtained for each marker in the case that the arranged at least one marker is a plurality of markers.
 3. A radiographic image processing method comprising: obtaining a plurality of radiographic images generated by arranging a plurality of detection unit that detect radiation that has been output from a radiation source and passed through a subject in such a manner that at least portions of the plurality of detection unit are overlapped with each other in an irradiation direction of the radiation, and by arranging at least one marker at a position to be detected by the plurality of detection unit, and by detecting the radiation that has passed through the subject and the marker by each of the plurality of detection unit; obtaining first information representing a distance between a detection surface of a first detection unit located closest to the radiation source among the plurality of detection unit and a detection surface of a second detection unit other than the first detection unit among the plurality of detection unit; obtaining second information representing a magnification ratio of a second marker image of the at least one marker included in a second radiographic image obtained by the second detection unit with respect to a first marker image of the at least one marker included in a first radiographic image obtained by the first detection unit; calculating, based on the first information and the second information, a radiation-source-to-image-surface distance, which is a distance between the radiation source and the detection surface of the first detection unit; performing scattered radiation removal processing on at least one of the plurality of radiographic images by using the radiation-source-to-image-surface distance; and performing energy subtraction processing on the plurality of radiographic images which include the at least one radiographic image on which the scattered radiation removal processing is performed.
 4. A non-transitory recording medium having recorded therein a radiographic image processing program that causes a computer to execute: a procedure that obtains a plurality of radiographic images generated by arranging a plurality of detection unit that detect radiation that has been output from a radiation source and passed through a subject in such a manner that at least portions of the plurality of detection unit are overlapped with each other in an irradiation direction of the radiation, and by arranging at least one marker at a position to be detected by the plurality of detection unit, and by detecting the radiation that has passed through the subject and the marker by each of the plurality of detection unit; a procedure that obtains first information representing a distance between a detection surface of a first detection unit located closest to the radiation source among the plurality of detection unit and a detection surface of a second detection unit other than the first detection unit among the plurality of detection unit; a procedure that obtains second information representing a magnification ratio of a second marker image of the at least one marker included in a second radiographic image obtained by the second detection unit with respect to a first marker image of the at least one marker included in a first radiographic image obtained by the first detection unit; a procedure that calculates, based on the first information and the second information, a radiation-source-to-image-surface distance, which is a distance between the radiation source and the detection surface of the first detection unit; a procedure that performs scattered radiation removal processing on at least one of the plurality of radiographic images by using the radiation-source-to-image-surface distance; and a procedure that performs energy subtraction processing on the plurality of radiographic images which include the at least one radiographic image on which the scattered radiation removal processing is performed. 